Electromagnetic waves in frequency bands of hundreds of megahertz to tens of gigahertz have hitherto been used by communication means. Frequency bands are distributed; for instance, a frequency band of 800 MHz (0.8 GHz) or a frequency band of 1.5 GHz band is allocated for mobile telephones; a frequency band of 1.9 GHz band is allocated for a personal handy phone system; a frequency band of 5.8 GHz band is allocated for an ETC (electronic toll collection) apparatus installed at a highway; a frequency band of 2.4 GHz band or 5.2 GHz band is allocated for a wireless LAN; and a frequency band of 5.8 GHz is allocated to DSRC (dedicated short range communication).
Electromagnetic waves in these frequency bands are utilized in connection with automobile operations, or such utilization is highly feasible. Plans are now afoot to receive these electromagnetic waves by use of a single antenna and to subject the received electromagnetic waves to digital processing, thereby collectively utilizing the electromagnetic waves. In such a case or a case where each of the electromagnetic waves in these frequency bands is solely used, there is required a band-pass filter which allows transmission of a signal in only a predetermined band among the frequency bands and blocks the other signals, in order to process data while blocking noise induced from harmonic waves or reflected waves.
In order to meet the demand, the present applicant has developed several electromagnetic shielding materials which are formed by dispersing powder of soft magnetic materials into a rubber or plastic matrix, and has put them into practical use.
One of the present inventors has already proposed a low-pass filter utilizing the electromagnetic-wave absorptive shielding material (as described in JP-A-2002-171104), and has also proposed a band-pass filter for gigahertz band to be used in frequency bands of hundreds of megahertz to tens of gigahertz by utilization of expertise in that low-pass filter (as described in JP-A-2004-222086).
FIGS. 4A and 4B shows an example of the thus-proposed band-pass filter for gigahertz band.
As shown in FIGS. 4A and 4B, a band-pass filter for gigahertz band 100 is formed by: placing an input signal line 102 and an output signal line 103 on the surface of a sheet 101 with an interval therebetween, the signal lines being formed form a conductor strip and running in a series direction; connecting mutually-opposing ends of the lines 102, 103 together with a chip capacitor 105 sandwiched therebetween; and placing a GND line 104 on the back of the sheet 101.
However, there is a problem of difficulty being encountered in miniaturizing the band-pass filter for gigahertz band.
In order to miniaturize the band-pass filter for gigahertz band 100, it is better to use a ceramic substrate of high complex relative permittivity for the sheet 101, which is a dielectric substrate, in such a way that a wavelength is compressed. However, when a ceramic substrate of high complex relative permittivity is used for the sheet 101, a connection of electromagnetic waves to the GND line 104 is interrupted, which leads to a failure to achieve a sufficient shielding effect. This raises another problem of deterioration of a rise characteristic and a fall characteristic.